Isolating molecules of pharmaceuticals, dyes, magnetic or optically sensitive particles or plant, animal or human tissues or cells as well as living organisms including viruses, bacteria, fungi, seeds or plant or animal embryos in the capsules not permitting transport of particles or bigger molecules through their walls with the possibility of controlled release of these objects through biocompatible chemical or biochemical treatment under ambient conditions opens tremendous perspectives in such domains as drug delivery, optical or magnetic storage of information through printing, sensor techniques, bio-delivery and even protection of biological objects, for the purpose of bio-control protection and/or preservation.
The earlier developed techniques for preparation of inorganic hydrosols and hydrogels for encapsulation purposes have been considering almost exclusively preparation of a silica gel. Two major approaches have been developed: one based on hydrolysis of silicon alkoxides using acid catalyst with subsequent addition of a buffer solution, stabilizing pH in the interval 5-7 [1, 2]. Silicon alkoxides are not soluble in water or miscible with it, which requires application of a co-solvent such as alcohol (not less than 30% in the total reaction mixture) or prolonged ultrasonic treatment or other mixing procedures to assure homogenization [1]. Other possibility is provided by hydrolysis of sodium silicate in water solution by addition of a buffer solution with pH in the interval 5-7 [3, 4]. In the latter case a considerable lack of reproducibility in encapsulation has been observed, caused by the difficulty to control local pH and provide a kinetically reproducible regime in the growth of a silica polymer [5]. Both these approaches offered polymeric gels, providing retention of the molecules or organisms trapped inside, but no true encapsulation, as these objects are released in a poorly controllable way through diffusion [2, 5]. Preparation of dense silica capsules without a possibility to release encapsulates has been reported to occur in water-in-oil [6] or oil-in-water [7] emulsions on application of stabilizing surfactants. Very recently it has been shown that heteroleptic silicon precursors, alkyl-silicon alkoxides, can give capsules, when hydrolyzed in an oil-in-water emulsion [8]. The use of metal alkoxides in the same procedure as silicon alkoxides has been claimed in [9], but was the same year shown to be impossible by the works of Livage et al. [10]. According to Livage, the hydrolysis of metal alkoxides is providing polymer sols that quickly transform into gels on addition of water. The possibility to obtain hydrosols for encapsulation purposes was proposed only through obtaining oxide gels in organic solvents and their re-peptization after transfer into water [5]. The measures proposed for obtaining such secondary sols, such as addition of strong acids or heating to at least 90° C. [11], are obviously not biocompatible. Recently [12] there has been reported preparation of core-shell Ag—TiO2 nanoparticles, applying hydrolysis of titanium isopropoxide solution in ethanol by a colloid solution of silver nanoparticles in water, stabilized by a surfactant CTAB (Cetylammonium bromide). Selective formation of the shell on the surface of the particles was attributed to the catalytic action of the surfactant.
It has recently been demonstrated [13] that the hydrolysis of the chemically modified zirconium and titanium alkoxides in a water-in-hydrocarbon emulsion (a system with phase separation between two solvents) occurred as a micellar self-assembly, providing thin-walled oxide shells selectively encapsulating hydrophilic molecules.